The regulation of chromium in the coating systems has created a new challenge for the coating technologists. State of the art chromate conversion coatings protect metal substrates, due to the ability of highly mobile hexavalent chromium to travel to sites of corrosion and reduce itself to trivalent chromium and adsorb irreversibly on the metal surface to inhibit cathodic and anodic reactions. Unfortunately the same ability of hexavalent chromium to reduce trivalent chromium leaves some molecular debris from transition steps that induce the critical changes in DNA which leads to lung cancer.
Researchers have taken various routes to make safer self-healing coatings. One route employs compounds similar to chromium such as molybdenum, cerium and vanadium. But the corrosion inhibition efficiency of these compounds is far less than chromium. Another approach relies on the encapsulation of reactive resins in the coatings. The reactive resins are contained in capsules which rupture with mechanical or pH stimulus and release the reactive resin that cures on contact with either a cross-linker or a catalyst. The cross-linker or catalyst is usually mixed previously in the matrix itself or can be contained in separate capsules, which are dispersed in the matrix.
This approach of incorporating capsules in the coatings also comes with its own limitations. Other than the effect on mechanical and physical properties of the coating, the low probability of presence of both reactants at the same time and in the required ratio at the damage site is a hindrance to successful usage of this technology.